A furnace may provide heat by burning fuel gas in a combustible mixture with oxidant gas. Such a furnace includes one or more burners, and has a reactant supply system with lines and valves that direct streams of the fuel and oxidant gases to the burners. In some furnaces the fuel and oxidant gases are provided to the burners in separate streams that form combustible mixtures within the burners. In other furnaces the fuel and oxidant gases are provided to the burners in a combustible mixture known as premix. In each case, the furnace has a control system that controls the reactant supply system so that combustion of the fuel will occur in a manner appropriate for the heating process to be performed by the furnace. The control system includes instruments such as temperature sensors, igniters, and flame detectors, and controls the valves in the reactant supply system with reference to combustion parameters indicated by those instruments.
Parts of a known furnace apparatus 10 are shown schematically in FIGS. 1 and 2. These include a burner assembly 12 mounted in a furnace wall structure 14. The wall structure 14 defines a combustion chamber 15, which may be referred to as a process chamber, with a flue gas outlet 16. A reactant supply system 20 provides reactants to the burner assembly 12 under the direction of a control system 22. This enables the burner assembly 12 to provide a controlled amount of heat for a heating process to be carried out in the combustion chamber 15.
The reactant supply system 20 includes fuel and oxidant supply lines 24 and 26 that convey fuel and oxidant from respective sources 28 and 30 to the burner assembly 12. The oxidant is typically the oxygen in a stream of air, and the fuel is typically natural gas. A motorized oxidant valve 32 controls the stream of air in the oxidant supply line 26. A pair of motorized fuel valves 34 and 36 control the stream of fuel in the fuel supply line 24. The oxidant valve 32 and the first fuel valve 34 are modulating valves. The second fuel valve 36 is a shut-off valve. Each valve motor is operated by the control system 22 to open, shift, and close the respective valve 32, 34 or 36 as directed by the control system 22.
The burner assembly 12 has two major portions 40 and 42 that are shown schematically in FIG. 1. The first portion 40 is a burner tile with an internal reaction zone 43. A generally cylindrical inner surface 44 of the burner tile 40 is centered on an axis 45, and defines the peripheral boundaries of the reaction zone 43. An outer end surface 46 of the burner tile 40 defines an outlet 47 that communicates the reaction zone 43 with the combustion chamber 15. The outlet 47 is circular and also is centered on the axis 45. An end portion 48 of the inner surface 44 is tapered radially inward so that the outlet 47 is constricted relative to the generally 10 cylindrical configuration of the reaction zone 43.
The second major portion 42 of the burner assembly 12 comprises a burner with a fuel inlet 50 and an oxidant inlet 52. Passages within the burner 42 receive the streams of fuel and oxidant from the inlets 50 and 52 to form a combustible mixture, and the burner 42 is oriented to fire a flame into the reaction zone 43 in a direction extending into the combustion chamber 15 through the outlet 47.
The control system 22 includes a controller 60. The control system 22 further includes a temperature sensor 62 which is operative in the combustion chamber 15. Additional parts of the control system 22 that are shown schematically in FIG. 1 include an igniter 64 and a flame detector 66. These are conventional devices that are operatively mounted in the burner assembly 12 in a known manner.
Other parts of the burner assembly 12, the furnace wall structure 14, the reactant supply system 20 and the control system 22 are known to a person of ordinary skill in the art. Those parts, such as a flame stabilizer within the burner assembly 12, are omitted from the drawings for clarity of the schematic illustrations.
As indicated in FIG. 1, the controller 60 includes flame supervisory controls in the form of hardware and/or software 70 for operating the furnace apparatus 10 with flame supervision, and further has a flame supervision bypass function, including hardware and/or software 72 for bypassing flame supervision. As the controller 60 carries out those instructions, it actuates the blower 30 and the valves 32, 34 and 36 to provide streams of fuel and oxidant along the supply lines 24 and 26, and actuates the igniter 64 to initiate combustion of those reactants in a flame that projects from the burner 42.
As the streams of fuel and oxidant continue to flow to the burner 42, the controller 60 provides flame supervision in accordance with the corresponding flame supervisory controls 70. Flame supervision is one of several supervisory functions the controller 60 performs by monitoring sensors that can indicate system malfunctions. Such sensors are known in the art and are omitted from the drawings for clarity of illustration.
If a malfunction occurs, the controller 60 can respond by closing the shut-off valve 36 as a safety precaution. If a malfunction does not present an unsafe condition, the controller 60 can bypass the supervisory function for the corresponding sensor, and can allow combustion to continue. The controller 60 thus monitors the flame detector 66 in readiness to close the shut-off valve 36 if the flame detector 66 indicates the absence of a flame. This would occur if the flame were inadvertently extinguished by a system malfunction. However, the controller 60 monitors the temperature sensor 62 also, and is operative to compare the sensed combustion chamber temperature to a predetermined auto-ignition temperature of the fuel supplied to the burner 42. If the sensed combustion chamber temperature is not less than the auto-ignition temperature, and if the flame supervision bypass function 72 is enabled, the flame supervisory controls 70 are bypassed. With the flame supervisory controls 70 bypassed, indication by the flame detector 66 of the absence of a flame projecting from the burner 42 will not result in the controller 60 closing the shut-off valve 36. Rather, the controller 60 then holds the shut-off valve 36 open so that the fuel can continue to flow through the burner 42 and onward through the reaction zone 43 to enter the combustion chamber 15 through the outlet 47. This results in diffuse combustion of the fuel upon auto-ignition in the combustion chamber 15 in the absence of a flame at the burner 42, as shown in FIG. 2.
As shown in FIGS. 3 and 4, the furnace apparatus 10 can be modified by providing a controller 80 with additional control functions 90 for initiating diffuse combustion in a controlled manner, i.e. for inducing diffuse combustion. The controller 80 of FIGS. 3 and 4 can be a modification of the prior art controller 60 described above, or a replacement for the prior art controller 60.
This controller 80 also commences combustion of the reactants with a flame at the burner 42, as shown in FIG. 3. However, unlike the prior art controller 60, the controller 80 can induce a diffuse mode of combustion. The additional control functions 90 can direct the controller 80 to induce diffuse combustion either automatically without intervention by an operator of the modified apparatus 10, or only if the operator provides corresponding actuating input. The additional control functions 90 could direct the controller 80 to receive the actuating input either before, during, or after the flame is ignited.
If diffuse combustion is to be induced, the controller 80 compares the sensed combustion chamber temperature with the predetermined auto-ignition temperature of the fuel to determine if the sensed temperature is below the auto-ignition temperature. This is preferably accomplished by comparing the combustion chamber temperature to a specified bypass setpoint temperature that is predetermined to be above the auto-ignition temperature of the fuel. If the combustion chamber temperature is less than the specified temperature, and if the flame detector 66 then indicates the absence of a flame at the burner 42, the controller 80 closes the shut-off valve 36. On the other hand, if the flame detector 66 indicates the continued presence of the flame, the controller 80 continues to monitor the sensed combustion chamber temperature with reference to the specified temperature. When the sensed combustion chamber temperature increases from a level below the specified temperature to a level at or above the specified temperature, the controller 80 responds by enabling the flame supervision bypass function 72 so that absence of a flame at the burner 42 will not result in closing of the shut-off valve 36. The controller 80 then begins to execute the diffuse combustion function 90, closing the shut-off valve 36. This extinguishes the flame at the burner 42.
The controller 80 continues to monitor the sensed combustion chamber temperature during a short delay, such as about five seconds, after closing the shut-off valve 36. This ensures that the flame has been fully extinguished, which can be confirmed by the flame detector 66. If the sensed combustion chamber temperature drops below the specified temperature during the delay, the shut-off valve 36 will remain closed until the process is restarted. However, if the sensed combustion chamber temperature remains at or above the specified temperature, as it was when the shut-off valve 36 was closed, the controller 80 will reopen the shut-off valve 36. The fuel stream will then flow once again through the burner 42 and onward through the reaction zone 43 to enter the combustion chamber 15 through the outlet 47. Although the igniter 64 is ordinarily actuated if the shut-off valve 36 is opened from a closed condition, it is not actuated in response to reopening of the shut-off valve 36 in this manner. Auto-ignition of the fuel then occurs in the combustion chamber 15 to provide diffuse combustion in the absence of a flame at the burner 42, as shown in FIG. 4. The controller 80 continues to monitor the sensed combustion chamber temperature during furnace operation in the diffuse combustion mode, and will close the shut-off valve 36 if the temperature falls below the specified temperature.
As described above, the flame is extinguished by closing the shut-off valve 36. Instead, the flame could be extinguished by operating the modulating valves 32 and 34, and/or the blower 30, so as to cause the reactant supply system 20 to provide the reactants to the burner assembly 12 in a fuel to oxidant ratio that does not sustain the flame at the burner 42. This could be accomplished by interrupting, decreasing or increasing the flow of oxidant to the burner 42 while maintaining the flow of fuel. Recirculated flue gas (FIG. 5) also could be introduced in the burner assembly 12 to extinguish the flame by diluting the fuel to oxidant ratio in a similar manner. The reactant supply system 20 can further be provided with a source of compressed air (FIG. 6), or an inert gas such as nitrogen, for creating a pulse which would extinguish the flame. Once the flame has been extinguished, the relatively lesser flow area provided by the restricted configuration of the reaction zone outlet 47 imparts correspondingly greater velocities to the reactants flowing through the reaction zone 43 and into the combustion chamber 15, and thus helps to ensure that the flame will not reignite.